Usually, a brushless motor needs a detector for detecting the position of magnetic poles of a rotor of the motor. However, for example, in the case where the brushless motor is used in a compressor for an air conditioner, it is not possible to use such a rotor pole position detector since the reliability of the detector cannot be ensured under the conditions of high temperature and high pressure. Accordingly, in such applications a method is employed which uses no pole position detector and in which a voltage signal induced in an armature winding is detected and a commutation signal of the motor is generated on the basis of the detected voltage signal. A rotating speed is detected on the basis of the commutation signal used to drive the motor while always making a feedback control in order to cope with variations in load, as shown in FIG. 1.
However, for example, in the case of a large-sized brushless motor for driving of a compressor of an air conditioner, there is a need to frequently change a rotating speed in accordance with the state of air-conditioning of the interior and exterior of a room. In such applications a so-called pulse width modulation scheme is usually used in which a voltage is controlled by controlling the duty of a voltage signal pulse with a DC power source voltage being kept constant.
Accordingly, the use of the conventional driving method mentioned above involves a problem that the change of a rotating speed takes a considerable time, thereby missing the effect of an urgent protection control.